Hand held surgical instruments are well known in the surgical community and have been used for centuries. Many of these instruments have been used for grasping, dissecting, cutting, ligating, or fastening objects to the body. Of interest are hand held surgical instruments that are used to grasp or manipulate tissue during a surgical procedure. Of special interest are hand held instruments that are adapted to operate laparoscopically, that is, in a minimally invasive surgical procedure wherein the surgery is performed through a small number of small diameter surgical access ports rather than through a large opening or incision within the patient. In a typical laparoscopic surgery, the abdominal cavity is insulated with an inert gas and surgical access ports are inserted into the patient. Laparoscopic surgical instruments are inserted into the access ports and the surgical procedure is performed through these access ports. Laparoscopic surgery is sometimes referred to as "keyhole surgery" wherein the access ports are the "keyholes" through which the surgery is performed. As a consequence of the access port ("keyhole") size, laparoscopic grasping instruments are characterized by a pair of scissor-like handles, a small diameter elongated shaft that forms a gas tight seal with the access port, and an end effector that is operatively coupled with at least one of the handles.
During open and laparoscopic surgery, it is important to provide surgical instruments capable of grasping and manipulating tissue. Grasping and holding tissue over long periods of time can be tiring and can cause medical complications should the surgeon tire and lose his grip upon the tissue. It is now traditional to provide a one-way holding or clamping mechanism on surgical grasping instruments to conserve the surgeon's stamina for other surgical tasks. Many such grasping instruments use a rack and pawl clamping mechanism operatively coupled to the end effector via the scissors-like handles. The rack and the pawl have sawtooth shaped teeth that have an angled side of the tooth and an undercut side of the tooth. The rack of the grasping instrument is operatively coupled to one handle and the pawl is defectively coupled to the other. The angled side of the rack and pawl teeth are brought into sliding engagement as the end effectors are moved together to clamp on tissue and the undercut side are brought into locking engagement when the end effectors attempt to move apart. The pawl must be disengaged from the rack and the pawl must be disengaged by a cam or other means to reopen the end effectors. One such instrument for an open procedure was described by D. M. Litlejohn in U.S. Pat. No. 1,659,112. Another instrument having a pawl and a rack mechanism operably coupled to scissors-like handles was described by Bales et al. in U.S. Pat. No. 5,176,702. The instrument described by Bales et al. is used in laparoscopic surgery.
Whereas the laparoscopic graspers described by Bales et al. locked when clamped upon tissue, they were difficult to use, as they required the use of a second hand to unlock the instrument during surgery. What was needed was a laparoscopic grasping instrument having a rack and pawl locking mechanism that can be easily locked and unlocked with the same hand that actuates the instrument. Laparoscopic grasping instruments having a pair of scissors-like handles and an easily accessible trigger for unlocking the pawl from the ratchet were disclosed by Green et al. in U.S. Pat. No. 5,476,479 and by Aranyi in U.S. Pat. No. 5,483,952.
The rack and pawl locking mechanism of the laparoscopic locking instruments disclosed by Green et al. and Aranyi has a limitation. The rack and pawl mechanism does not provide infinite positions during closure but is limited to a number of discreet positions that are dependent on the distance between the rack teeth. Since the pawl must fall between two rack teeth to lock, the size of the of the rack tooth limits the number of discreet locking positions. If the tooth profile is large, the surgeon is limited to a very few discrete locking positions and the risk of clamping the tissue too tight or too loose is increased. If the tooth profile is small, the number of discreet locking positions is increased, reducing, but not eliminating, the limitations of a toothed locking mechanism.
What is needed is a locking mechanism that has an infinite number of locking positions as the handles are closed. Such a mechanism is described by T. C. Hutcheson in U.S. Pat. No. 804,229, Nicholas in U.S. Pat. No. 5,314,424, and Measamer et al. in U.S. Pat. No. 5,735,874. The locking mechanism used by Hutcheson, Nicholas, and Measamer is a plate having an aperture or hole therethrough and pivotable upon a shaft extending through the hole. The end effectors are operably coupled to the actuation handle by the shaft. When the plate is angled with respect to the longitudinal axis of the shaft, the hole becomes elliptical, contacts the shaft, and forms a one way frictional lock with the shaft. When the plate is positioned at right angles to the shaft, the actuation handle is unlocked as the hole in the plate is circular relative to the shaft and the shaft slips freely through the hole in both directions.
The inventions of Hutcheson and Measamer were indeed revolutionary, but the actuation buttons or triggers were confusing to use when actuating or unactuating the one way slip locking mechanism. The actuation mechanism for Nicholas was also confusing. What is needed is an infinitely variable one-way locking mechanism that is releasable and intuitive to use.